Conventional methods of transferring and casting liquid metals by pouring effectively in free fall under gravity practically always introduce defects into the liquid because of the turbulent folding over of the liquid surface. Such transfer folds the surface oxide into the bulk of the liquid metal. These folded films possess no bonding between the opposed oxide surfaces, and so act as cracks in the liquid. The freezing-in of the doubled-over films (known as ‘bifilms’) into castings results in poor and erratic mechanical properties and low fatigue resistance of the cast component. The bifilms are frozen into products solidified from the melt. They lower both the strength and ductility of such products.
Once introduced, bifilms are not easily eliminated from liquid metals, particularly oxide bifilms in liquid aluminum (Al) and its alloys. The reason for this is that the bifilm initially ravels into a compact ball, allowing it to pass through most filters. Later, in the casting, it can unravel, becoming a serious crack-like defect that reduces the properties of the casting. Bifilms in Al and its alloys will also not sink or float in any reasonable time. The reason for this is that the aluminum oxide is slightly denser than liquid Al and so should sink. However, when folded in by surface turbulence (for instance during a pouring action) the air entrained between the folded-over film (giving it its name ‘bifilm’ to denote its doubled-over nature) causes it to float. This nearly neutral buoyancy together with its film-like aspect ratio confers an extremely low Stokes velocity. Thus any slight convection in the melt will cause the suspended oxide defects to circulate for hours or days. The problem of bifilms is especially serious in light metals, such as magnesium and aluminum and in alloys which contain these elements.
It is beginning to be widely understood therefore that quiescent transfers of the melt, avoiding the “bucket technology” approach that is so common in the industry, are necessary for good results. However, the quest for totally quiescent melting, and totally quiescent transfers to effect casting, has not been easy, and has thus far been elusive. First, the melt needs to be obtained. One way of obtaining the melt is via a dry hearth furnace. In the known dry hearth melter designs, the dry hearth furnace empties into an intermediate holding bath where additional burners heat the metal up to casting temperature. Thereafter, such systems pour the metal or hand ladle or robot ladle the melt from the intermediate reheater of the dry hearth furnace into the holder or other onward transfer systems.
Prior to casting, the cleaning of the metal is usually nowadays carried out by the production of thousands or millions of minute bubbles of an inert gas introduced beneath the surface of the melt. This can be achieved via porous plugs inserted into the walls or base of the holding vessel, or by rotary degassing, in which an immersed rotor is caused to spin in the liquid, releasing clouds of minute bubbles. Such techniques use inert gas to flush out dissolved hydrogen and usually operate with impressive efficiency. Furthermore, it is thought that the bubbles attach to suspended oxides and carry them to the surface of the melt, from where they can be skimmed off. Unfortunately, however, this technique also re-introduces millions of minute double oxide films because the inert gas cannot be truly inert; it will always contain sufficient contaminating gases to create a thin oxide layer on the surface of every bubble. In addition, of course, the bursting of the bubbles at the surface of the melt necessarily reveals the interior of the bubble to the air, so increasing the possibilities for the re-introduction of oxide defects.
Thus the so-called cleaning process is one in which the few large double oxides are probably replaced by millions of small double oxides. For this reason, the total cleaning action is less than optimum. Even worse, it is not uncommon for rotary degassers to operate in such a way that a vortex is formed around the rotor shaft which carries air down into the melt, re-introducing oxides as fast as they can be removed. In addition to these problems, if the rotor assembly is not completely dry (the refractories are likely to have absorbed up to ten percent water vapor over a weekend for instance) the first several minutes of operation of the rotary degasser will cause an increase in the gas content of the melt. Thus the melt will get worse before it gets better. For those systems working on an automatically-controlled (i.e. rather short) degassing time, the final result is likely to be a melt with increased rather than decreased hydrogen content. Also, of course, the general stirring actions in the liquid that any kind of bubble degassing introduces to the melt, eliminates any chance of inclusions separating via a sink or float process.
Moving on to the problems of casting, the transferring of the molten metal into the mold cavity, there has been considerable interest in the use of pneumatic dosing systems. However, the embodiment of such systems has so far involved the use of furnaces that are large pressurized vessels. These are not easily controlled because of their large volume of compressible gas, and large amount of heavy liquid that needs to be accelerated into the mold cavity. Even more seriously, such units have to be filled with liquid metal, and the filling is usually carried out by pouring under gravity, often from a considerable height, thus introducing the very defects that the process seeks to avoid. There is, thus, good reason to avoid surface turbulence at every stage of melting and casting of the molten metal if this is possible.
Only in the last few years has it become clear how to handle liquid aluminum alloys and other liquid metals. The fundamental issue involved is the problem of entrainment. This problem has rarely been identified as a source of concern for the liquid metal industries, especially in the shaped casting industry, where pouring of metals is endemic to the industry. Of course, the pouring of a molten metal or melt is the worst type of handling, causing maximum entrainment.
Furthermore, having suffered some degree of entrainment, it has not been clear what to do about it. The settling of entrained oxides has been known to a few foundry operators, such as is discussed in U.S. Pat. No. 4,967,827 dated Nov. 6, 1990. The '827 patent discloses the Cosworth Casting Process, in which the melt is made by an unspecified melting process, held for some time in a large holding furnace of conventional design, and casting is carried out by the use of an electromagnetic pump. However, the holding furnace is of such a shape, with its large depth, that the cooler walls encourage downward flow of cooler metal, setting up a convective stirring regime that prevents the efficient settling of inclusions in suspension. No one appreciated that the settling process was woefully inefficient, because of convection, in the rather deep furnaces employed, such as is illustrated in the '827 patent. The problem with such furnaces is that the sides of such a furnace are always at a different temperature than are the tops and bottoms of the furnace.
Moreover, even if one has successfully detrained or removed defects from the liquid metal or melt, few have connected such settling tanks directly to a counter gravity filling system, such as is illustrated in the '827 patent. This, at least, avoids the reintroduction of freshly entrained defects, such as via pouring or other turbulence. Even here, however, the holding furnace was connected to an extremely expensive electromagnetic pump. The existing electro-magnetic pumps are also somewhat counter-productive because of the huge power dissipation in the working volume of the pumps, resulting in very high redundant forces which cause intense high velocity stirring; only a minute fraction of the electrical power is used in the useful propulsion and pressurization of the metal. At the present time, despite the useful and praiseworthy historical advance that this system represents, neither the melting, nor holding, nor casting technology is in accordance with the recommendations of the present disclosure. The current disclosure seeks to improve greatly on this previous top world-class casting process. It will be understood, of course, that the principles disclosed herein apply not only to metals such as aluminum and magnesium but apply equally well to a wide range of metals and alloys.
Other previous attempts at improved melting and casting processes can be mentioned that also generally fall short of the advantages noted in the current disclosure. It is noteworthy that the geometry of the holding vessels is not described in any detail in the prior art documents. Thus the central issue of quiescent holding to the degree that suppression of convection is achieved, encouraging inclusions of nearly neutral buoyancy to settle under gravity, is not appreciated in the prior art.
Accordingly, it has been considered desirable to develop a new and improved quiescent transfer process for molten metals which would overcome the foregoing difficulties and others while producing better and more advantageous results. It would also be beneficial to employ a simple and low cost system for a filling operation of a mold.